The present invention pertains to a recyclable-fuel engine system, and in particular to a system adapted for use with a motor vehicle.
The feasibility of using a recyclable-fuel engine system in a motor vehicle has been studied by the Exxon Research and Engineering Company, Linden, N.J., under Environmental Protection Agency contract #68-02-2135. A copy of the Exxon study is available from the National Technical Information Service, U.S. Department of Commerce, Springfield, Va., 22161, as technical bulletin TEC-75/003. The system proposed by the Exxon Company includes an internal combustion engine and a catalytic reactor which is supplied heat from engine exhaust gases. The catalytic reactor is a shell and tube type unit whose plural tubular bed surfaces are coated with a suitable catalyst, such as platinum. When supplied with heat, the catalyst functions to catalyze the dehydrogenation of a saturated carrier hydrocarbon, such as methylcyclohexane to hydrogen and an unsaturated form of the carrier, such as toluene. The hydrogen produced is used to power the engine, with the unsaturated form of the carrier being returned to the fuel tank, to be replaced, periodically, with saturated carrier.
A significant problem brought out in the Exxon study is that a catalytic reactor of the type proposed therein would require a warmup period in excess of about twenty minutes before reaching operative catalytic temperature. During the warmup period, the engine would have to be supplied stored hydrogen, requiring heavy on-board pressure storage tanks in a vehicle. Another problem identified in the Exxon study is the requirement for excess hydrogen used to supply heat to the catalytic reactor after reactor warmup, to compensate for the endothermicity of the dehydrogenation reaction used to produce the hydrogen fuel in the vehicle. The Exxon study estimates that the amount of heat supplied to the reactor from engine exhaust gases would be about two-thirds of the total heat needed to drive the catalytic reaction at a "self-sustaining" hydrogen production level. The study concludes that nearly half of the total available fuel in the system would be used in supplying additional heat to the catalytic unit, either during warmup or during normal catalytic reactor operation. The additional fuel weight and hydrogen storage tank weight thus required seriously limit the efficiency of the proposed Exxon system.
Another problem which is anticipated in a recyclable-fuel engine system using dehydrogenation catalysis is a gradual loss in reactor catalytic activity due to the deposition of breakdown fragments on the catalytic bed. In the usual large-scale catalytic dehydrogenation operation, this problem is solved by performing the catalytic reaction at elevated hydrogen pressure, typically between 10 and 20 atm. Such pressure is suitable, of course, in commercial applications where an ample supply of hydrogen and energy to drive a pressure-generating pump are available, and where the weight of such pump and the required pressure-containment system is not critical. However, the same factors make the high-pressure system inappropriate for use in a vehicle.
There is a further consideration which makes the use of a high pressure system impractical for vehicle use. As is known, most organic lubricants become unstable above 400.degree. C., thus placing an upper limit on the temperature which the engine block is allowed to reach. For the hydrocarbon carrier molecules proposed herein, the temperature at which the dehydrogenation reaction is strongly favored, between about 1 and 6 atm, is 380.degree. C., or lower. At a pressure of between 10 and 20 atm, the temperature required to produce the same degree of dehyrogenation may be as high as 430.degree. C. Thus, it can be appreciated how a high pressure dehydrogenation system would produce intractable engine lubrication problems.
A general object of the present invention is to provide a recyclable-fuel engine system which solves or minimizes the above-discussed problems in recyclable-fuel engine systems adapted for use in a vehicle.
An important specific object of the invention is to provide, in a recyclable-fuel system, a catalytic unit designed to catalyze the dehydrogenation of a hydrocarbon carrier at low pressure, and apparatus for periodically regenerating the catalytic activity of the unit's catalyst.
Another object of the present invention is to provide, in a recyclable-fuel system including a catalytic unit and an internal combustion engine having combustion and exhaust chambers, heat transfer from both of these chambers to the unit.
Specifically, and related to the above, it is an object to provide in such a system, a plurality of heat pipes for transferring heat from the engine's combustion and exhaust chambers to the catalytic unit.
Yet another object of the invention is to provide, in a recyclable-fuel engine system, a catalytic unit which includes a plurality of catalytic beds, at least one of which can be brought to catalytic temperature independently of the other beds.
The present invention is a recyclable-fuel engine system which includes a hydrogen-utilizing internal combustion engine and a hydrogen-producing catalytic unit. The engine includes combustion and exhaust chambers which are heated by combustion and exhaust gases, respectively. The catalytic unit includes an enclosure housing at least a pair of catalytic beds, each of which functions, when supplied with heat, to catalyze the reaction of a reduced form of a hydrocarbon carrier to hydrogen and a dehydrogenated form of the carrier. Heat may be transferred directly from the exhaust chamber to one of the beds within the catalytic unit, to supply the same with heat during catalyst warm-up. The remainder catalytic beds are supplied heat by heat pipes connecting the combustion and exhaust chambers thereto.
The dehydrogenation reaction occurring within the catalytic unit is performed at a temperature of between 300.degree. and 380.degree. C., at a pressure of about one atmosphere of hydrogen. Under these reaction conditions, there occurs a deposit of carbonaceous material, including fragments of the carrier, on the catalytic beds, reducing the catalytic activity thereof. The deposit is oxidized, to regenerate the catalytic activity of the beds, by periodically and selectively evacuating the catalytic unit enclosure and introducing air thereinto.